A radio-controlled timepiece receiving time information conveyed by a carrier wave having a predetermined frequency to adjust its own time based on that time information has been finding various applications such as clocks, wristwatches, etc.
The radio waves used for the radio-controlled timepieces, etc. are 40-200 kHz, having as long wavelengths as several kilometers. Because antennas as long as more than several hundred meters are needed to efficiently receive these radio waves, it is practically difficult to use them in wristwatches, keyless entry systems, RFID systems, etc. Accordingly, magnetic cores having the same function as that of the antennas are generally used for receiving radio waves.
Two radio waves of 40 kHz and 60 kHz are used as carrier waves for time information in Japan. Radio waves having frequencies of 100 kHz or less are mainly used overseas to provide time information. To receive radio waves of these frequencies, magnetic sensor-type antennas having coils wound around magnetic cores are mainly used.
A wristwatch is mainly constituted by a housing, a movement (driver module) and its peripheral parts (dial, motor, battery, etc.), a non-metal (glass) cover, and a rear metal cover. When an antenna is contained in a wristwatch, it is conventionally disposed outside the housing in many cases.
However, the recent trend of reducing size and weight has required an antenna to be disposed in a housing. FIG. 23 shows one example of wristwatches containing an antenna in a housing. As shown in FIG. 23, it should be noted that a movement 92, a rear cover 94, and peripheral parts 96 such as a battery, a motor for moving a pointer, etc. are disposed in a housing 95, and an antenna 1 is placed in a gap between the movement 92 and the rear cover 94. Though the antenna 1 is shown by a solid line in the front view of FIG. 23, the antenna 1 is contained in a closed space defined by the housing 95, the movement 92, the peripheral parts 96 and the rear cover 94. Thus, the antenna 1 is not actually seen from front.
When a radio wave coming from outside passes through a magnetic core, voltage is induced in a coil. As shown in the equivalent circuit of FIG. 22, this voltage resonates at a desired frequency by a coil 8 and a capacitor C connected to the coil 8 in parallel. A Q-times voltage is generated in the coil 8 by resonance, to cause current to flow. This resonance current causes the coil 8 to generate a magnetic field, whose magnetic flux mainly flows in and out of both ends of the magnetic core. If there is a metal around the antenna, the magnetic flux generated by this resonance current penetrates the metal, generating eddy current. Thus, if there is a metal near the antenna, the energy of a magnetic field is lost as eddy current at the time of resonance, resulting in antenna coil loss and thus decrease in a Q value and antenna sensitivity.
JP 2003-110341 A discloses a small antenna comprising a magnetic core constituted by an amorphous metal laminate, and a coil wound around it. JP 8-271659 A discloses a small antenna comprising a magnetic core made of ferrite and a coil wound around it. These small antennas are disposed mainly outside the housings of the wristwatches. From the aspect of not hindering the receiving of radio waves as described above, a wristwatch comprising the antenna described in JP 2003-110341 A or JP 8-271659 A preferably has a resin housing.
However, the resin housing poses restrictions in design and ornamentation. Generally, design is a selling point for wristwatches, and metal housings are preferred for high-quality impression and beautifulness. Accordingly, most high-quality timepieces have metal housings. However, if the small antenna described in JP 2003-110341 A or JP 8-271659 A is mounted in a wristwatch with a metal housing the metal housing acts as a radio wave shield, resulting in drastic reduction of receiving sensitivity.
JP 2002-168978 A discloses an antenna comprising a conductive seal member between a metal housing and an antenna. The antenna of this reference is disposed outside the metal housing via a shield member to keep a Q value. However, because the seal member is indispensable, it suffers restrictions in size reduction and design.
Japanese Patent 3,512,782 discloses an antenna comprising a magnetic main path member comprising a coil wound around a magnetic core, and a magnetic sub-path member comprising a magnetic core without a coil, an air gap being provided in part of a closed magnetic loop along the magnetic core, such that a magnetic flux generated inside at the time of resonance less leaks outside. Japanese Patent 3,512,782 describes that this antenna selectively guides a magnetic flux flowing outward at the time of resonance to the magnetic sub-path member, thereby making the magnetic flux less likely to leak outside to suppress the reduction of a Q value due to an eddy current loss.
Keyless entry systems and RFID systems also suffer the problem that a metal hinders an antenna from transmitting and receiving radio waves. The keyless entry system and the RFID system also contain a magnetic sensor-type antenna disposed in a metal housing or near metal parts. The keyless entry system capable of doing the remote control of an automobile key, etc. comprises a receiving unit having an antenna for doing a switching operation by a particular electromagnetic wave, and a unit for transmitting an electromagnetic wave. When a key holder, a transmitting unit, goes close to or away from the receiving unit, a door can be opened or closed without touching the key. The RFID (radio frequency identification) system gives and receives information stored in a tag through an antenna operated at a particular electromagnetic wave. For instance, when an RFID tag, to which destination information, etc. are input, is mounted to a bus, etc., and when an RFID tag, to which timetable information is input, is embedded in a display board, etc. at a bus stop, various transportation information can be seen. In these systems, too, the size reduction and sensitivity increase of an antenna are required.